This invention relates to powered door actuating systems and, more particularly, to an electrical actuating system for industrial type refrigerators and coolers or similar container doors.
Industrial refrigerators are customarily equipped with large sliding doors weighing several hundred of pounds which require some force of power drive, most often electro-mechanical. In prior art arrangements, an electric motor of sufficient capacity is often used to power a direct drive mechanical linkage which actuates the door to open or close it. It is often the case that such linkage is both cumbersome and complicated necessitating frequent maintenance. More importantly, because the refrigerator door is generally quite heavy, the motor is subjected to overloads when initiating movement of the door because of the high inertia thereof, and, in addition, it has proved difficult to prevent sudden and sharp jerks as the door is moved or stopped, thereby placing unduly large strains on components of the actuating system.
Efforts to minimize such drawbacks have included the use of a mechanical clutch between the motor and the linkage, wherein the clutch itself is capable of absorbing some of the shock through slippage, but such efforts have not been entirely successful or reliable, inasmuch as reliance on clutch slippage per se requires that slippage be predictable and reliably and uniformly reproducible.
In the prior art, as embodied in U.S. Pat. Nos. 3,132,728 of the Chaptois, 4,623,052 of Watanabe et .al, and 4,624,349 of Watanabe, some of the drawbacks of mechanical clutches are avoided through the use of magnetic clutches. These prior art patents disclose the use of magnetic clutches in conjunction with automotive vehicle drive trains in which the degree of magnetization of the clutch is tied to the speed or torque of the automobile motor, and, by so doing, a soft start characteristic is achieved. In all of these patents, the circuitry or mechanical linkage involved in making the energization of the clutch a function of motor speed or torque is quite complicated, albeit a necessary evil in the disclosed arrangements.